This invention relates to a light emitting device and its application, more particularly, to an improved ultraviolet light emitting device and its application in which light emission is made preferentialy at a desired wave length for optimum deposition with good stability.
A low pressure mercury lamp has been broadly used in the field of semiconductors due to its high performance of ultraviolet light emission. Especially, photo CVD for forming semiconductor layers on a substrate is advantageous to carry out the deposition at a relatively low temperature with chemical reaction taking place under the existence of ultraviolet radiation. The photo CVD method is superior to CVD methods of other types, e.g., thermal CVD or plasma CVD, in that the deposition will not injure the surface of a substrate on which a layer is deposited.
One example of such a photo CVD apparatus is shown in FIG. 1. The apparatus comprises a reaction chamber 2 in which substrates 1 are disposed, a heater 3 for heating the substrates 1 to a predetermined temperature, a low pressure mercury lamp 9 for irradiating the substrates 1 with ultraviolet light through a light window 16, a reactant gas supply system 7 for supplying reaction gas to the reaction chamber 2 through a mercury bubbler 13 and a exhausting system provided with a rotary pump 19. In the chamber 2, a chemical reaction with reactant gas introduced from the supply system 7 sets up, and the gas component is decomposed by virtue of ultraviolet light depositing thereby on the substrates 1 the decomposed product, e.g., an amorphous silicon layer at 250.degree. C. The amorphous silicon layer, however, is deposited also on the light window 16 which normally made of quartz. To prevent the silicon layer from being deposited, the window is coated with Fomblin oil 16 (a Trademark of an oil of fluoride composition).
A problem of the prior art is a low deposition speed. One measure to solve the above problem is the provision of the mercury bubbler 13 which doses mercury vapor into the reactant gas as a sensitizer. The mercury vapor, in turn, has a potential to be involved in pollution problem. Thus, it would be most desirable in the semiconductor field to prepare a deposition method which can form a semiconductor layer at high speed without the arising pollution problem.
The deposition speed largely depends on the ultraviolet light incident on the reaction gas from the light source. The chemical reaction producing a material to be deposited is set up with radiation from the mercury lamp 9. Accordingly, the wavelength and the intensity of the light are of most importance to make the deposition efficient.
The mercury lamp as a light source is generally comprised of a bulb which contained an amount of mercury gas at several torrs mixed with argon gas. Opposed ends of the bulb are provided with electrodes between which electron discharge is to take place. Within the discharge, argon atoms and mercury atoms are supplied with their exciting energy through collision with electrons.
Referring to FIG. 2, there are illustrated some energy levels of mercury and argon which are referred to in this description. In this graphical diagram, 3.sup.1 P.sub.1 represents the energy state having a principal quantum number of 3, an orbital type of "p", a spin multiplied degree of 1 in the presuperscript and a total magnetic momentum 1 in the subscript. Transitions among the levels take place in chemical vapor deposition according to scatterings of two types. One type is optical scattering type concerning an interaction between electromagnetic field and an electron of the atom. The other is elastic scattering type concerning an interaction between an electron of the atom and an electron from the electrode. The former gives rise to ultraviolet light emission on the basis of transition between two levels only with spin even, while the later causes an energy exchange between the electrons without forbidden transitions but according to the scattering cross section shown in FIG. 5.
According to experiments, mercury gas is easily exited when dosed with argon gas, compared to mercury gas alone. Further, the dose of argon gas also has a function to lessen scattering effect of the argon atoms on the discharge electrodes. Thus, argon gas is commercially dosed to mercury gas so as to initiate the discharge easily even under somewhat low energy supply with a low mercury pressure. The emission, however, takes place mainly at wave length of 254 nm as shown in FIG. 4. The inventors have conceived a hypothesis to explain the role of the dosed argon gas in the CVD action as follows. Namely, since having the large scattering cross section, argon atoms receive energy of electrons emitted from the electrodes more effectively than mercury atoms and change in their energy states from the ground level to the exited levels ELa. The excited argon atoms in turn collide with and render their energy to mercury atoms, pumping up same to the energy levels .sup.3 P.sub.0, .sup.3 P.sub.1 and .sup.3 P.sub.2 from the ground level. Although the transitions between one of the three levels and the ground state are forbidden due to spin preservation in the first order apploximation, the transition of second order, which permit emission of light with 254 nm wave length, are observed where a major part of the excited states are occupied by electrons. On the other hand, the scattering cross section to the .sup.1 P.sub.1 level is not so large especially in low energy region as shown in FIG. 5. Because of this, the prior art device has emitted mainly on the basis of the transition of electrons from the triplet levels, namely with the vicinity of 254 nm in wave length. In the semiconductor process, however, ultraviolet light with about 185 nm in wave length is favored rather than with 254 nm, especially when decomposition of silane (Si.sub. n H.sub.2n+2) is desired.